Total Variation RUMBA for FOD optimization

[rutgerfick]

Alternative FOD optimizer taking into account non-Gaussian noise and enabling total variation spatial regularization.

• Should click into the MultiCompartmentSphericalHarmonics model, but would need to be adapted to allow for iterating over the entire image for the total variation term.
• Perhaps TV should be an option for any MC framework in Dmipy.

Canales-Rodríguez, Erick J., et al. "Spherical deconvolution of multichannel diffusion MRI data with non-Gaussian noise models and spatial regularization." PloS one 10.10 (2015): e0138910.